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3o6                        FUELS  (GENERAL METHODS)

ordinary practical purposes, sufficiently exact results are obtained if the
corrections are omitted. Under such circumstances the thermometric
readings during the preliminary period and those after the maximum tem-
perature has been passed, and also the titration, becomes useless, the only
values required being those of the magnitudes in the expression,

«).....     (ii),

which then gives the calorific power.

It must also be mentioned that, whilst in the bomb the water (hygroscopic
water plus that formed by combustion of the hydrogen in the fuel) remains
in the liquid state, in practice it passes off as vapour among the products of
combustion ; consequently, the calorific power calculated as above includes
also the heat of condensation of the water, which in practice is not utilisable.
In France the calorific power resulting from the above calculation, that is,
presupposing the formation of liquid water (also called gross calorific power) is
given, whereas in Germany, Austria, and elsewhere, the heat of condensation
of the water is deducted, the assumption being made that the water remains
as vapour (net calorific power). Taking 6co cals. as the heat of condensation
of i gram of aqueous vapour, if H and M are the percentages of hydrogen and
moisture in the fuel, the deduction to be made from the gross calorific power
to obtain the net value is 6 (M + gH). Where an elementary analysis is not
made, a separate determination may be made of the total water evolved during
the combustion (by burning a given weight of fuel in a tube and collecting the
water In an absorption apparatus), or, as Mahler suggests, in practical cases
mean "values of H may be taken according to the quality of the fuel tested.

EXAMPLE :   The experimental data obtained were as follows :

A = 2200 grams.
a =    474
= 0-125


/  = 0-032

Temperature observed

0  minutes

1        ,,


	7 minutes 8


15-200 15-205 (ignition)

15795 17-850


18-305  (max.






Hence T1 — T = 18-305 — 15-205 = 3-100°.    The law of variation before

ignition is given by

15-205 — 15-180              0

_j£ — ^ - 1 - = 0-005°,

and that after the maximum is passed by

— 18-230               _

£_ =0-015°.

The correction to be made for the first half-minute after ignition is hence

~°'005-=— 0-0025° I for the next half-minute it is°'°15 ~ 0'°°5 • = 0-005,
2                                                                                        2

and for each of the minutes, 6-7 and 7-8, it is 0-015.    Hence
t = 2 x 0-015 + 0*005 — 0-0025 = 0*0325.thout taking account of the heat absorbed by the